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The issue is fixed in placeholderadmin, but the 'cancel' button logic is duplicated with the one in pageadmin. However, the 'edit_plugin' (and other plugin handling methods) already has most of it's logic duplicated in the two modules. I could refactor the code and remove all duplication, but the changeset would be quite massive and would probably require some design decisions.

Besides the issue, also improved test coverage and fixed a couple of miss behaving tests.